This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statement of this solution are to be read in this light and are not to be understood as admission about what is in the prior art or what is not in the prior art.
For 3GPP work item Further Evolution Carrier Aggregation (FeCA), up to 32 downlink (DL) carriers shall be supported for downlink transmission. The HARQ feedback (ACK/NACK) for DL data transmission is linearly increased compared to Rel-10 where there are only 5 DL Component Carriers (CC) at most and the resource for transmission of HARQ feedback is reserved for each configured CC. However, the carrier aggregation of FeCA may have up to 32 CCs and it is possible that a few licensed CCs are aggregated with many unlicensed CCs.
The use of LTE Carrier Aggregation (CA), introduced in Rel-10 and enhanced in Rel-11, offers means to increase the peak data rates, system capacity and user experience by aggregating radio resources from multiple carriers that may reside in the same band or different bands and, in the case of inter-band TDD CA, may be configured with different Uplink (UL)/DL configurations. In Rel-12, carrier aggregation between TDD and FDD serving cells is introduced to support UE connecting to them simultaneously.
In Rel-13, LAA (Licensed-Assisted Access) has attracted a lot of interests in extending the LTE carrier aggregation feature towards capturing the spectrum opportunities of unlicensed spectrum in the 5 GHz band. WLAN operating in the 5 GHz band nowadays already supports 80 MHz in the field and 160 MHz is to follow in Wave 2 deployment of IEEE 802.11ac. There are also other frequency bands, such as 3.5 GHz, where aggregation of more than one carrier on the same band is possible, in addition to the bands already widely in use for LTE. Enabling the utilization of at least similar bandwidths for LTE in combination with LAA as IEEE 802.11ac Wave 2 deployment, will support calls for extending the carrier aggregation framework to support more than 5 carriers. The extension of the CA framework beyond 5 carriers has been approved to be one work item for LTE Rel-13. The objective is to support up to 32 carriers in both UL and DL.
Compared to single-carrier operation, a UE operating with CA has to report HARQ feedback for more than one DL component carriers. Meanwhile, a UE does not have to support DL and UL CA simultaneously. For instance, the first release of CA capable UEs in the market only supports DL CA but not UL CA. This is also the underlying assumption in the 3GPP RAN4 standardization. Therefore, an enhanced UL control channel, i.e. PUCCH format 3 was introduced for CA during Rel-10 timeframe. However, in order to support more DL component carriers in Rel-13 for those UE not support UL CA, the UL control channel capacity becomes a limitation.
According to current HARQ protocol, ACK/NACK shall be reported for each downlink data transmission. For FDD, there are up to 64 HARQ ACK/NACK bits at one time (Rank>=2) for up to 32 DL CC, depending on the number of configured DL CCs. For TDD, the number of HARQ-ACK/NACK bits depends on the number of configured CCs and UUDL subframe configuration of the DL CCs. Assuming that there are 32 DL CCs with UUDL subframe configuration 2 and transmission mode 3, there are up to 256 (32*4*2) HARQ ACK/NACK bits. Assuming that ½ coding rate and QPSK modulation are applied, in FDD scenario, it needs 32 REs (assuming that spatial bundling is applied) at least, while in TDD scenario, it needs 128 REs (assuming that spatial bundling is applied) at least.
For FeCA, ideally up to 32 CCs can be configured for one UE. But it is less likely that all of the DL CCs are available for one UE due to a number of reasons, for example: within the up to 32 CCs, there may be a huge number of unlicensed CCs which are shared between different networks, for instance, the unlicensed carrier over 5 GHz band can be shared by co-existing WiFi networks and multiple co-existing LTE networks; or there are many users served by the same cell and the carrier resources shall be divided between the served users; or one FeCA capable user may need less than the configured CC sometimes due to the traffic variations.
As mentioned above, unlicensed CCs are shared between multiple operators and sparsely scheduled CCs for one UE can be the frequent case. According to the existing mechanism for transmission of HARQ feedback, the UE needs to reserve the resource for each configured CCs, and this may result in unendurable overhead for transmission of HARQ feedback.